1. Field of the Invention
The present invention relates to a method for changing, particularly reducing the mold width during continuous casting of molten metal in particular.
2. Description of the Prior Arts
Conventional methods for reducing the mold width during continuous casting have generally been that both the upper and lower ends are moved simultaneously and at the same speed, in other words, the wall face of the mold is moved while being maintained to be parallel to the solid shell of the molten slab in the mold before the width changing.
The conventional methods have the following disadvantages and defects.
According to the conventional methods, when the moving speed of the mold wall is increased the deformation resistance of the solid shell of the molten slab also increases. Therefore, in order to perform the mold width reduction stably and consistently, it would be necessary to provide a width changing device having a pushing force larger than the above deformation resistance of the solid shell of the molten slab, hence the maximum mold wall movement is necessarily determined by the capacity of the width changing device, and the actual mold wall movement speed cannot exceed this maximum mold wall movement speed.
Also according to the conventional methods, the width reduction is performed by depressing and deforming the solid shell of the slab by the inside wall surface of the mold so that the inside wall surface is subjected to increase wear and resultant slabs are more susceptible to crackings to be caused in the same direction of the oscillation marks on the shorter sides of the slabs.
The conventional methods are illustrated in FIGS. 1a, 1b and 1c, in which the deformation amount of the solid shell of the slab caused by the movement of the inside wall of the mold is expressed as .DELTA.W=ut, and the maximum deformation amount is expressed by .DELTA.W.sub.max =uL/v.
______________________________________ For illustration, examples are shown as below: L v u Calculated .DELTA.W.sub.max ______________________________________ 800 mm 600 mm/min. 4 mm/min 5.3 mm 2 mm/min 2.7 mm ______________________________________
In FIGS. 1a, 1b and 1c 1 represents a mold wall, 2 represents a solid shell of the slab and 3 represents molten steel in the mold. u represents the mold width changing speed, namely the mold wall moving speed, v represents the casting speed, L represents the effective length of the mold and t represents the time after the commencement of the width changing.
Another conventional method as disclosed in Japanese Patent Application Laid-Open No. Sho 50-152926 comprises the following steps for performing the mold width reduction from a preceding mold width to a subsequent mold width during continuous casting.
(1) The upper end portion of the narrow side wall of the mold is inwardly moved in proportion to the slab drawing speed so as to reduce the width of the upper surface of the molten steel in the mold.
(2) The upper end portion of the narrow side wall is further moved until a desired width of the upper surface of the molten steel in the mold can be obtained.
(3) During the movement of the upper end portion, the lower end portion of the narrow side wall is pressed against the slab, and moves inwardly and gradually as the slab is reduced in width.
(4) Then the upper end portion of the narrow side wall of the mold is maintained so as to obtain a constant width of the upper surface of the molten steel in the mold, and while the lower end portion is pressed, the slab is drawn and the narrow side wall is maintained at the new position to complete the width change of the mold.
As understood from the sequential steps of the above conventional method, only the upper end portion of the narrow side wall of the mold is positively moved toward the center of the mold until a desired dimension of the upper surface of the molten steel can be obtained, while the lower end portion is pressed with an appropriate force against the slab, and is gradually and inwardly moved as the slab is reduced in width. Thus, the process of the conventional method is repetition of the pattern that the lower end portion of the narrow side wall of the mold is passively moved in pursuance of the changes in the slab width caused by the movement of the upper end portion of the narrow side wall, and after the upper end portion of the mold has reached a desired dimension, the upper end portion of the narrow side wall is maintained there and only the lower end portion of the narrow side wall is positively moved toward the center of the mold to complete the width changing (reduction). This is clearly different from the changing pattern of the mold cross section.
Further, according to the above conventional method, the lower end portion of the narrow side wall is pressed by appropriate force all the time during the width changing operation so as to follow up the changes in the upper surface of the molten steel, but this method cannot be said as a practically satisfactory process in view of the forming condition of the solid shell of the molten slab during continuous casting, and is very likely to be susceptible to operational difficulties such as break-out.